Shizuku and Seiji fom whisper of the heart.
I also have to second Usui and Misaki.
Ryuuki and Shurei from Saiunkoku. Actually most of the pairings in Saiunkoku rock: Yuushun and Rin, Eigetsu and Kourin, Shunki and Kokujun, and I dare not forget Reishin and Kijin!!